Genetic correction of a LRRK2 mutation in human iPSCs links parkinsonian neurodegeneration to ERK-dependent changes in gene expression

C9ORF72 repeat expansion causes vulnerability of motor neurons to Ca2+-permeable AMPA receptor-mediated excitotoxicity

Mutations in C9ORF72 are the most common cause of familial amyotrophic lateral sclerosis (ALS). Here, through a combination of RNA-Seq and electrophysiological studies on induced pluripotent stem cell (iPSC)-derived motor neurons (MNs), we show that increased expression of GluA1 AMPA receptor (AMPAR) subunit occurs in MNs with C9ORF72 mutations that leads to increased Ca²⁺-permeable AMPAR expression and results in enhanced selective MN vulnerability to excitotoxicity. These deficits are not found in iPSC-derived cortical neurons and are abolished by CRISPR/Cas9-mediated correction of the C9ORF72 repeat expansion in MNs. We also demonstrate that MN-specific dysregulation of AMPAR expression is also present in C9ORF72 patient post-mortem material. We therefore present multiple lines of evidence for the specific upregulation of GluA1 subunits in human mutant C9ORF72 MNs that could lead to a potential pathogenic excitotoxic mechanism in ALS.

Repeat expansion mutation in C9ORF72 is the most common cause of familial ALS. Here, the authors generate motor neurons from cells of patients with C9ORF72 mutations, and characterize changes in gene expression in these motor neurons compared to genetically corrected lines, which suggest that glutamate receptor subunit GluA1 is dysregulated in this form of ALS.

Impaired DNA damage response signaling by FUS-NLS mutations leads to neurodegeneration and FUS aggregate formation

Amyotrophic lateral sclerosis (ALS) is the most frequent motor neuron disease. Cytoplasmic fused in sarcoma (FUS) aggregates are pathological hallmarks of FUS-ALS. Proper shuttling between the nucleus and cytoplasm is essential for physiological cell function. However, the initial event in the pathophysiology of FUS-ALS remains enigmatic. Using human induced pluripotent stem cell (hiPSCs)-derived motor neurons (MNs), we show that impairment of poly(ADP-ribose) polymerase (PARP)-dependent DNA damage response (DDR) signaling due to mutations in the FUS nuclear localization sequence (NLS) induces additional cytoplasmic FUS mislocalization which in turn results in neurodegeneration and FUS aggregate formation. Our work suggests that a key pathophysiologic event in ALS is upstream of aggregate formation. Targeting DDR signaling could lead to novel therapeutic routes for ameliorating ALS.

Abnormal cytoplasmic aggregates of FUS are a hallmark of some forms of amyotrophic lateral sclerosis (ALS). Here, using neurons derived from patients with FUS-ALS, the authors demonstrate that impairment of PARP-dependent DNA damage signaling is an event that occurs upstream of neurodegeneration and cytoplasmic aggregate formation in FUS-ALS.

Reprogramming neurodegeneration in the big data era

Recent genome-wide association studies (GWAS) have identified numerous genetic risk variants for late-onset Alzheimer's disease (AD) and Parkinson's disease (PD). However, deciphering the functional consequences of GWAS data is challenging due to a lack of reliable model systems to study the genetic variants that are often of low penetrance and non-coding identities. Pluripotent stem cell (PSC) technologies offer unprecedented opportunities for molecular phenotyping of GWAS variants in human neurons and microglia. Moreover, rapid technological advances in whole-genome RNA-sequencing and epigenome mapping fuel comprehensive and unbiased investigations of molecular alterations in PSC-derived disease models. Here, we review and discuss how integrated studies that utilize PSC technologies and genome-wide approaches may bring new mechanistic insight into the pathogenesis of AD and PD.

Anti-TNF-alpha antibody attenuates subarachnoid hemorrhage-induced apoptosis in the hypothalamus by inhibiting the activation of Erk

BACKGROUND

Subarachnoid hemorrhage (SAH) can induce apoptosis in many regions of the brain including the cortex and hippocampus. However, few studies have focused on apoptosis in the hypothalamus after SAH. Although some antiapoptotic strategies have been developed for SAH, such as anti-tumor necrosis factor-alpha (TNF-α) antibody, the molecular mechanisms underlying this condition have yet to be elucidated. Therefore, the purpose of this study was to evaluate whether SAH could induce apoptosis in the hypothalamus and identify the potential molecular mechanisms underlying the actions of anti-TNF-α antibody, as a therapeutic regimen, upon apoptosis.

MATERIALS AND METHODS

SAH was induced in a rat model. Thirty minutes prior to SAH, anti-TNF-α antibody or U0126, an extracellular signal-regulated kinase (Erk) inhibitor, was microinjected into the left lateral cerebral ventricle. In addition, phorbol-12-myristate-13-acetate was injected intraperitoneally immediately after the anti-TNF-α antibody microinjection. Then, real-time polymerase chain reaction, Western blotting and immunohistochemistry were used to detect the expression of caspase-3, bax, bcl-2, phosphorylated Erk (p-Erk) and Erk. Finally, anxiety-like behavior was identified by using open field.

RESULTS

Levels of caspase-3, bax and bcl-2, all showed a temporary rise after SAH in the hypothalamus, indicating the induction of apoptosis in this brain region. Interestingly, we found that the microinjection of anti-TNF-α antibody could selectively block the elevated levels of bax, suggesting the potential role of anti-TNF-α antibody in the inhibition of SAH-induced apoptosis in the hypothalamus. Moreover, we found that Erk activation was necessary for apoptosis after SAH and that the microinfusion of anti-TNF-α antibody could inhibit apoptosis by suppressing the increase of p-Erk in the hypothalamus. Finally, our data indicated that the infusion of anti-TNF-α antibody could improve anxiety-like behavior.

CONCLUSION

Collectively, our data demonstrate that anti-TNF-α antibody attenuates apoptosis in the hypothalamus by inhibiting the activation of Erk, which plays an important role in the treatment of SAH.

Induced Pluripotent Stem Cells in Disease Modeling and Gene Identification

Experimental modeling of human inherited disorders provides insight into the cellular and molecular mechanisms involved, and the underlying genetic component influencing, the disease phenotype. The breakthrough development of induced pluripotent stem cell (iPSC) technology represents a quantum leap in experimental modeling of human diseases, providing investigators with a self-renewing and, thus, unlimited source of pluripotent cells for targeted differentiation. In principle, the entire range of cell types found in the human body can be interrogated using an iPSC approach. Therefore, iPSC technology, and the increasingly refined abilities to differentiate iPSCs into disease-relevant target cells, has far-reaching implications for understanding disease pathophysiology, identifying disease-causing genes, and developing more precise therapeutics, including advances in regenerative medicine. In this chapter, we discuss the technological perspectives and recent developments in the application of patient-derived iPSC lines for human disease modeling and disease gene identification.

Overexpression of UHRF2 in intrahepatic cholangiocarcinoma and its clinical significance

Ubiquitin-like with PHD and ring finger domains 2 (UHRF2) has been implicated in tumorigenesis. However, its roles in intrahepatic cholangiocarcinoma (ICC) are still unclear. In this study, UHRF2 expression was analyzed in several kinds of cancers by referring to public Oncomine database, and the levels of UHRF2 mRNA and protein were determined in ICC cells and tissues. Then, the roles of UHRF2 in ICC were investigated by UHRF2 interference. Moreover, the relationship between UHRF2 and E-cadherin expression was examined in ICC cells and samples. Finally, the prognostic role of UHRF2 in ICC was analyzed in 139 ICC patients by Cox regression and Kaplan–Meier methods. We found UHRF2 was overexpressed in multiple human cancers, as well as in ICC, and the invasion, migration, proliferation, and antiapoptosis of ICC cells were inhibited by UHRF2 interference. Moreover, the epithelial–mesenchymal transition-related marker E-cadherin was upregulated in ICC cells which was influenced by UHRF2 expression. Clinically, UHRF2 expression was positively associated with microvascular invasion and lymphatic metastasis of ICC, and patients in the UHRF2^high group had much lower overall survival and higher recurrence rates than patients in the UHRF2^low group. A multivariate analysis revealed that UHRF2 overexpression was a new prognostic marker for ICC. Thus, our results indicated that high level of UHRF2 might be a novel predictor for the prognosis of ICC.

Decreased Sirtuin Deacetylase Activity in LRRK2 G2019S iPSC-Derived Dopaminergic Neurons

Mitochondrial changes have long been implicated in the pathogenesis of Parkinson's disease (PD). The glycine to serine mutation (G2019S) in leucine-rich repeat kinase 2 (LRRK2) is the most common genetic cause for PD and has been shown to impair mitochondrial function and morphology in multiple model systems. We analyzed mitochondrial function in LRRK2 G2019S induced pluripotent stem cell (iPSC)-derived neurons to determine whether the G2019S mutation elicits similar mitochondrial deficits among central and peripheral nervous system neuron subtypes. LRRK2 G2019S iPSC-derived dopaminergic neuron cultures displayed unique abnormalities in mitochondrial distribution and trafficking, which corresponded to reduced sirtuin deacetylase activity and nicotinamide adenine dinucleotide levels despite increased sirtuin levels. These data indicate that mitochondrial deficits in the context of LRRK2 G2019S are not a global phenomenon and point to distinct sirtuin and bioenergetic deficiencies intrinsic to dopaminergic neurons, which may underlie dopaminergic neuron loss in PD.

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LRRK2 G2019S iPSC-derived dopaminergic neurons have unique mitochondrial defects

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LRRK2 G2019S dopaminergic neurons have increased sirtuin levels but reduced activity

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LRRK2 G2019S dopaminergic neurons have reduced NAD⁺ levels compared with other neurons

Understanding Parkinson's Disease through the Use of Cell Reprogramming

Recent progress in the field of somatic cell reprogramming offers exciting new possibilities for the study and treatment of Parkinson's disease (PD). Reprogramming technology offers the ability to untangle the diverse contributing risk factors for PD, such as ageing, genetics and environmental toxins. In order to gain novel insights into such a complex disease, cell-based models of PD should represent, as closely as possible, aged human dopaminergic neurons of the substantia nigra. However, the generation of high yields of functionally mature, authentic ventral midbrain dopamine (vmDA) neurons has not been easy to achieve. Furthermore, ensuring cells represent aged rather than embryonic neurons has presented a significant challenge. To date, induced pluripotent stem (iPS) cells have received much attention for modelling PD. Nonetheless, direct reprogramming strategies (either to a neuronal or neural stem/progenitor fate) represent a valid alternative that are yet to be extensively explored. Direct reprogramming is faster and more efficient than iPS cell reprogramming, and appears to conserve age-related markers. At present, however, protocols aiming to derive authentic, mature vmDA neurons by direct reprogramming of adult human somatic cells are sorely lacking. This review will discuss the strategies that have been employed to generate vmDA neurons and their potential for the study and treatment of PD.

Induced Pluripotent Stem Cells in Huntington's Disease Research: Progress and Opportunity

Induced pluripotent stem cells (iPSCs) derived from controls and patients can act as a starting point for in vitro differentiation into human brain cells for discovery of novel targets and treatments for human disease without the same ethical limitations posed by embryonic stem cells. Numerous groups have successfully produced and characterized Huntington’s disease (HD) iPSCs with different CAG repeat lengths, including cells from patients with one or two HD alleles. HD iPSCs and the neural cell types derived from them recapitulate some disease phenotypes found in both human patients and animal models. Although these discoveries are encouraging, the use of iPSCs for cutting edge and reproducible research has been limited due to some of the inherent problems with cell lines and the technological differences in the way laboratories use them. The goal of this review is to summarize the current state of the HD iPSC field, and to highlight some of the issues that need to be addressed to maximize their potential as research tools.

Mitochondrial Calcium Dysregulation Contributes to Dendrite Degeneration Mediated by PD/LBD-Associated LRRK2 Mutants

Mutations in leucine-rich repeat kinase 2 (LRRK2) contribute to development of late-onset familial Parkinson's disease (PD), with clinical features of motor and cognitive dysfunction indistinguishable from sporadic PD. Calcium dysregulation plays an important role in PD pathogenesis, but the mechanisms of neurodegeneration remain unclear. Recent reports indicate enhanced excitatory neurotransmission in cortical neurons expressing mutant LRRK2, which occurs before the well-characterized phenotype of dendritic shortening. As mitochondria play a major role in the rapid buffering of cytosolic calcium, we hypothesized that altered mitochondrial calcium handling contributes to dendritic retraction elicited by the LRRK2-G2019S and -R1441C mutations. In primary mouse cortical neurons, we observed increased depolarization-induced mitochondrial calcium uptake. We found that expression of mutant LRRK2 elicited transcriptional upregulation of the mitochondrial calcium uniporter (MCU) and the mitochondrial calcium uptake 1 protein (MICU1) with no change in levels of the mitochondrial calcium antiporter NCLX. Elevated MCU and MICU1 were also observed in LRRK2-mutated patient fibroblasts, along with increased mitochondrial calcium uptake, and in postmortem brains of sporadic PD/PDD patients of both sexes. Transcriptional upregulation of MCU and MICU1 was caused by activation of the ERK1/2 (MAPK3/1) pathway. Inhibiting ERK1/2 conferred protection against mutant LRRK2-induced neurite shortening. Pharmacological inhibitors or RNAi knockdown of MCU attenuated mitochondrial calcium uptake and dendritic/neuritic shortening elicited by mutant LRRK2, whereas expression of a constitutively active mutant of NCLX that enhances calcium export from mitochondria was neuroprotective. These data suggest that an increased susceptibility to mitochondrial calcium dysregulation contributes to dendritic injury in mutant LRRK2 pathogenesis.SIGNIFICANCE STATEMENT Cognitive dysfunction and dementia are common features of Parkinson's disease (PD), causing significant disability. Mutations in LRRK2 represent the most common known genetic cause of PD. We found that PD-linked LRRK2 mutations increased dendritic and mitochondrial calcium uptake in cortical neurons and familial PD patient fibroblasts, accompanied by increased expression of the mitochondrial calcium transporter MCU. Blocking the ERK1/2-dependent upregulation of MCU conferred protection against mutant LRRK2-elicited dendrite shortening, as did inhibiting MCU-mediated calcium import. Conversely, stimulating the export of calcium from mitochondria was also neuroprotective. These results implicate increased susceptibility to mitochondrial calcium overload in LRRK2-driven neurodegeneration, and suggest possible interventions that may slow the progression of cognitive dysfunction in PD.

The therapeutic potential of cell identity reprogramming for the treatment of aging-related neurodegenerative disorders

Neural cell identity reprogramming strategies aim to treat age-related neurodegenerative disorders with newly induced neurons that regenerate neural architecture and functional circuits in vivo. The isolation and neural differentiation of pluripotent embryonic stem cells provided the first in vitro models of human neurodegenerative disease. Investigation into the molecular mechanisms underlying stem cell pluripotency revealed that somatic cells could be reprogrammed to induced pluripotent stem cells (iPSCs) and these cells could be used to model Alzheimer disease, amyotrophic lateral sclerosis, Huntington disease, and Parkinson disease. Additional neural precursor and direct transdifferentiation strategies further enabled the induction of diverse neural linages and neuron subtypes both in vitro and in vivo. In this review, we highlight neural induction strategies that utilize stem cells, iPSCs, and lineage reprogramming to model or treat age-related neurodegenerative diseases, as well as, the clinical challenges related to neural transplantation and in vivo reprogramming strategies.

From the Cover: Manganese and Rotenone-Induced Oxidative Stress Signatures Differ in iPSC-Derived Human Dopamine Neurons

Parkinson's disease (PD) is the result of complex interactions between genetic and environmental factors. Two chemically distinct environmental stressors relevant to PD are the metal manganese and the pesticide rotenone. Both are thought to exert neurotoxicity at least in part via oxidative stress resulting from impaired mitochondrial activity. Identifying shared mechanism of action may reveal clues towards an understanding of the mechanisms underlying PD pathogenesis. Here we compare the effects of manganese and rotenone in human-induced pluripotent stem cells-derived postmitotic mesencephalic dopamine neurons by assessing several different oxidative stress endpoints. Manganese, but not rotenone caused a concentration and time-dependent increase in intracellular reactive oxygen/nitrogen species measured by quantifying the fluorescence of oxidized chloromethyl 2',7'-dichlorodihydrofluorescein diacetate (DCF) assay. In contrast, rotenone but not manganese caused an increase in cellular isoprostane levels, an indicator of lipid peroxidation. Manganese and rotenone both caused an initial decrease in cellular reduced glutathione; however, glutathione levels remained low in neurons treated with rotenone for 24 h but recovered in manganese-exposed cells. Neurite length, a sensitive indicator of overall neuronal health was adversely affected by rotenone, but not manganese. Thus, our observations suggest that the cellular oxidative stress evoked by these 2 agents is distinct yielding unique oxidative stress signatures across outcome measures. The protective effect of rasagiline, a compound used in the clinic for PD, had negligible impact on any of oxidative stress outcome measures except a subtle significant decrease in manganese-dependent production of reactive oxygen/nitrogen species detected by the DCF assay.

Transflammation: Innate immune signaling in nuclear reprogramming

Induction of pluripotency in somatic cells by retroviral overexpression of four transcription factors has revolutionized the field of stem cell biology and regenerative medicine. The efficient induction of pluripotency requires the activation of innate immune signaling in a process termed "transflammation" (Lee et al., 2012). Specifically, the stimulation of pattern recognition receptors (PRRs) causes global alterations in the expression and activity of epigenetic modifiers to favor an open chromatin configuration. Activation of toll-like receptors (TLR) or RIG-1-like receptors (RLR) (Sayed et al. 2017) trigger signaling cascades that result in NFκB or IRF-3 mediated changes in epigenetic plasticity that facilitate reprogramming. Another form of nuclear reprogramming is so-called direct reprogramming or transdifferentiation of one somatic cell to another lineage. We have shown that transdifferentiation of human fibroblasts to endothelial cells also involves transflammation (Sayed et al., 2015). Recently, we also identified reactive oxygen species (ROS) (Zhou et al. 2016) and reactive nitrogen species (RNS) (Meng et al., 2016) as mediators of innate immune signaling in nuclear reprogramming. Innate immune signaling plays a key role in nuclear reprogramming by regulating DNA accessibility (Fig. 1). Here, we review recent progress of innate immunity signaling in nuclear reprogramming and epigenetic plasticity.

Human Induced Pluripotent Stem Cells and the Modelling of Alzheimer's Disease: The Human Brain Outside the Dish

BACKGROUND

Neurodegenerative diseases like Alzheimer's Disease (AD) are a global health issue primarily in the elderly. Although AD has been investigated using primary cultures, animal models and post-mortem human brain tissues, there are currently no effective treatments.

SUMMARY

With the advent of induced pluripotent stem cells (iPSCs) reprogrammed from fully differentiated adult cells such as skin fibroblasts, newer opportunities have arisen to study the pathophysiology of many diseases in more depth. It is envisioned that iPSCs could be used as a powerful tool for neurodegenerative disease modelling and eventually be an unlimited source for cell replacement therapy. This paper provides an overview of; the contribution of iPSCs towards modeling and understanding AD pathogenesis, the novel human/mouse chimeric models in elucidating current AD pathogenesis hypotheses, the possible use of iPSCs in drug screening, and perspectives on possible future directions.

KEY MESSAGES

Human/mouse chimeric models using iPSCs to study AD offer much promise in better replicating AD pathology and can be further exploited to elucidate disease pathogenesis with regards to the neuroinflammation hypothesis of AD.

Stem Cell Technology for (Epi)genetic Brain Disorders

Despite the enormous efforts of the scientific community over the years, effective therapeutics for many (epi)genetic brain disorders remain unidentified. The common and persistent failures to translate preclinical findings into clinical success are partially attributed to the limited efficiency of current disease models. Although animal and cellular models have substantially improved our knowledge of the pathological processes involved in these disorders, human brain research has generally been hampered by a lack of satisfactory humanized model systems. This, together with our incomplete knowledge of the multifactorial causes in the majority of these disorders, as well as a thorough understanding of associated (epi)genetic alterations, has been impeding progress in gaining more mechanistic insights from translational studies. Over the last years, however, stem cell technology has been offering an alternative approach to study and treat human brain disorders. Owing to this technology, we are now able to obtain a theoretically inexhaustible source of human neural cells and precursors in vitro that offer a platform for disease modeling and the establishment of therapeutic interventions. In addition to the potential to increase our general understanding of how (epi)genetic alterations contribute to the pathology of brain disorders, stem cells and derivatives allow for high-throughput drugs and toxicity testing, and provide a cell source for transplant therapies in regenerative medicine. In the current chapter, we will demonstrate the validity of human stem cell-based models and address the utility of other stem cell-based applications for several human brain disorders with multifactorial and (epi)genetic bases, including Parkinson's disease (PD), Alzheimer's disease (AD), fragile X syndrome (FXS), Angelman syndrome (AS), Prader-Willi syndrome (PWS), and Rett syndrome (RTT).

Leucine-rich repeat kinase 2 exacerbates neuronal cytotoxicity through phosphorylation of histone deacetylase 3 and histone deacetylation

Parkinson's disease (PD) is characterized by slow, progressive degeneration of dopaminergic neurons in the substantia nigra. The cause of neuronal death in PD is largely unknown, but several genetic loci, including leucine-rich repeat kinase 2 (LRRK2), have been identified. LRRK2 has guanosine triphosphatase (GTPase) and kinase activities, and mutations in LRRK2 are the major cause of autosomal-dominant familial PD. Histone deacetylases (HDACs) remove acetyl groups from lysine residues on histone tails, promoting transcriptional repression via condensation of chromatin. Here, we demonstrate that LRRK2 binds to and directly phosphorylates HDAC3 at Ser-424, thereby stimulating HDAC activity. Specifically, LRRK2 promoted the deacetylation of Lys-5 and Lys-12 on histone H4, causing repression of gene transcription. Moreover, LRRK2 stimulated nuclear translocation of HDAC3 via the phoshorylation of karyopherin subunit α2 and α6. HDAC3 phosphorylation and its nuclear translocation were increased in response to 6-hydroxydopamine (6-OHDA) treatment. LRRK2 also inhibited myocyte-specific enhancer factor 2D activity, which is required for neuronal survival. LRRK2 ultimately promoted 6-OHDA-induced cell death via positive modulation of HDAC3. These findings suggest that LRRK2 affects epigenetic histone modification and neuronal survival by facilitating HDAC3 activity and regulating its localization.

Stem cells: disease models that show and tell

Combining gene-editing techniques with induced pluripotent stem cells is both powerful and tricky.

Brain imaging genetics in ADHD and beyond - Mapping pathways from gene to disorder at different levels of complexity

Attention-deficit/hyperactivity disorder (ADHD) is a common and often persistent neurodevelopmental disorder. Beyond gene-finding, neurobiological parameters, such as brain structure, connectivity, and function, have been used to link genetic variation to ADHD symptomatology. We performed a systematic review of brain imaging genetics studies involving 62 ADHD candidate genes in childhood and adult ADHD cohorts. Fifty-one eligible research articles described studies of 13 ADHD candidate genes. Almost exclusively, single genetic variants were studied, mostly focussing on dopamine-related genes. While promising results have been reported, imaging genetics studies are thus far hampered by methodological differences in study design and analysis methodology, as well as limited sample sizes. Beyond reviewing imaging genetics studies, we also discuss the need for complementary approaches at multiple levels of biological complexity and emphasize the importance of combining and integrating findings across levels for a better understanding of biological pathways from gene to disease. These may include multi-modal imaging genetics studies, bioinformatic analyses, and functional analyses of cell and animal models.

Modeling Glaucoma: Retinal Ganglion Cells Generated from Induced Pluripotent Stem Cells of Patients with SIX6 Risk Allele Show Developmental Abnormalities

Glaucoma represents a group of multifactorial diseases with a unifying pathology of progressive retinal ganglion cell (RGC) degeneration, causing irreversible vision loss. To test the hypothesis that RGCs are intrinsically vulnerable in glaucoma, we have developed an in vitro model using the SIX6 risk allele carrying glaucoma patient-specific induced pluripotent stem cells (iPSCs) for generating functional RGCs. Here, we demonstrate that the efficiency of RGC generation by SIX6 risk allele iPSCs is significantly lower than iPSCs-derived from healthy, age- and sex-matched controls. The decrease in the number of RGC generation is accompanied by repressed developmental expression of RGC regulatory genes. The SIX6 risk allele RGCs display short and simple neurites, reduced expression of guidance molecules, and immature electrophysiological signature. In addition, these cells have higher expression of glaucoma-associated genes, CDKN2A and CDKN2B, suggesting an early onset of the disease phenotype. Consistent with the developmental abnormalities, the SIX6 risk allele RGCs display global dysregulation of genes which map on developmentally relevant biological processes for RGC differentiation and signaling pathways such as mammalian target of rapamycin that integrate diverse functions for differentiation, metabolism, and survival. The results suggest that SIX6 influences different stages of RGC differentiation and their survival; therefore, alteration in SIX6 function due to the risk allele may lead to cellular and molecular abnormalities. These abnormalities, if carried into adulthood, may make RGCs vulnerable in glaucoma. Stem Cells 2017;35:2239-2252.

Neuronal Dysfunction in iPSC-Derived Medium Spiny Neurons from Chorea-Acanthocytosis Patients Is Reversed by Src Kinase Inhibition and F-Actin Stabilization

Chorea-acanthocytosis (ChAc) is a fatal neurological disorder characterized by red blood cell acanthocytes and striatal neurodegeneration. Recently, severe cell membrane disturbances based on depolymerized cortical actin and an elevated Lyn kinase activity in erythrocytes from ChAc patients were identified. How this contributes to the mechanism of neurodegeneration is still unknown. To gain insight into the pathophysiology, we established a ChAc patient-derived induced pluripotent stem cell model and an efficient differentiation protocol providing a large population of human striatal medium spiny neurons (MSNs), the main target of neurodegeneration in ChAc. Patient-derived MSNs displayed enhanced neurite outgrowth and ramification, whereas synaptic density was similar to controls. Electrophysiological analysis revealed a pathologically elevated synaptic activity in ChAc MSNs. Treatment with the F-actin stabilizer phallacidin or the Src kinase inhibitor PP2 resulted in the significant reduction of disinhibited synaptic currents to healthy control levels, suggesting a Src kinase- and actin-dependent mechanism. This was underlined by increased G/F-actin ratios and elevated Lyn kinase activity in patient-derived MSNs. These data indicate that F-actin stabilization and Src kinase inhibition represent potential therapeutic targets in ChAc that may restore neuronal function.

SIGNIFICANCE STATEMENT

Chorea-acanthocytosis (ChAc) is a fatal neurodegenerative disease without a known cure. To gain pathophysiological insight, we newly established a human in vitro model using skin biopsies from ChAc patients to generate disease-specific induced pluripotent stem cells (iPSCs) and developed an efficient iPSC differentiation protocol providing striatal medium spiny neurons. Using patch-clamp electrophysiology, we detected a pathologically enhanced synaptic activity in ChAc neurons. Healthy control levels of synaptic activity could be restored by treatment of ChAc neurons with the F-actin stabilizer phallacidin and the Src kinase inhibitor PP2. Because Src kinases are involved in bridging the membrane to the actin cytoskeleton by membrane protein phosphorylation, our data suggest an actin-dependent mechanism of this dysfunctional phenotype and potential treatment targets in ChAc.

CRISPR/Cas9 and piggyBac-mediated footprint-free LRRK2-G2019S knock-in reveals neuronal complexity phenotypes and α-Synuclein modulation in dopaminergic neurons

The p.G2019S mutation of the leucine-rich repeat kinase 2 (LRRK2) has been identified as the most prevalent genetic cause of familial and sporadic Parkinson's disease (PD). The Cre-LoxP recombination system has been used to correct the LRRK2-G2019S mutation in patient derived human induced pluripotent stem cells (hiPSCs) in order to generate isogenic controls. However, the remaining LoxP site can influence gene expression. In this study, we report the generation of a footprint-free LRRK2-G2019S isogenic hiPS cell line edited with the CRISPR/Cas9 and piggyBac technologies. We observed that the percentage of Tyrosine Hydroxylase (TH) positive neurons with a total neurite length of >2000μm was significantly reduced in LRRK2-G2019S dopaminergic (DA) neurons. The average branch number in LRRK2-G2019S DA neurons was also decreased. In addition, we have shown that in vitro TH positive neurons with a total neurite length of >2000μm were positive for Serine 129 phosphorylated (S129P) alpha-Synuclein (αS) and we hypothesize that S129P-αS plays a role in the maintenance or formation of long neurites. In summary, our footprint-free LRRK2-G2019S isogenic cell lines allow standardized, genetic background independent, in vitro PD modeling and provide new insights into the role of LRRK2-G2019S and S129P-αS in the pathogenesis of PD.

Induced Pluripotent Stem Cells Meet Genome Editing

It is extremely rare for a single experiment to be so impactful and timely that it shapes and forecasts the experiments of the next decade. Here, we review how two such experiments-the generation of human induced pluripotent stem cells (iPSCs) and the development of CRISPR/Cas9 technology-have fundamentally reshaped our approach to biomedical research, stem cell biology, and human genetics. We will also highlight the previous knowledge that iPSC and CRISPR/Cas9 technologies were built on as this groundwork demonstrated the need for solutions and the benefits that these technologies provided and set the stage for their success.

CADPS2 gene expression is oppositely regulated by LRRK2 and alpha-synuclein

The Ca²⁺-dependent activator protein for secretion 2 (CADPS2) is a member of the CAPS/CADPS protein family that plays crucial roles in synaptic vesicle dynamics. Genomic variability in the CADPS2 gene has been associated to autism spectrum disorders and Alzheimer's disease, both characterized by altered neurotransmission. Biological evidence also linked CADPS2 to Parkinson's disease (PD), as a disease-causing mutation in leucine-rich repeat kinase 2 (LRRK2) was reported to increase CADPS2 gene and protein expression. Furthermore, restoration of CADPS2 physiologic levels was able to provide neuroprotection in patient-derived neurons, consistent with the synaptic dysfunction postulated to underlie PD. However, little is known about the influence of PD-related proteins on transcriptional regulation of critical synaptic genes such as CADPS2. Here we aimed at investigating the transcriptional effects of LRRK2 and alpha-synuclein (aSyn) on CADPS2 gene expression, using a combination of in silico analyses and cell biology techniques. First, we identified a predicted promoter in the human CADPS2 genomic sequence, which we then utilized in a luciferase-based gene reporter assay. This approach enabled us to disclose a differential effect of high levels of LRRK2 and aSyn on CADPS2 promoter activity. Specifically, CADPS2 transcriptional activity was enhanced by high cellular levels of LRRK2 and reduced by overexpression of aSyn. Consistently, CADPS2 mRNA levels were diminished in aSyn overexpressing cells. Our results indicate that LRRK2 and aSyn participate in the dysregulation of CADPS2 by altering transcription and support the hypothesis that synaptic dysfunctions, through different mechanisms, might contribute to the neuronal defects of diseases such as PD.

Induced pluripotent stem cell-based modeling of neurodegenerative diseases: a focus on autophagy

The advent of cell reprogramming has enabled the generation of induced pluripotent stem cells (iPSCs) from patient skin fibroblasts or blood cells and their subsequent differentiation into tissue-specific cells, including neurons and glia. This approach can be used to recapitulate disease-specific phenotypes in classical cell culture paradigms and thus represents an invaluable asset for disease modeling and drug validation in the framework of personalized medicine. The autophagy pathway is a ubiquitous eukaryotic degradation and recycling system, which relies on lysosomal degradation of unwanted and potentially cytotoxic components. The relevance of autophagy in the pathogenesis of neurodegenerative diseases is underlined by the observation that disease-linked genetic variants of susceptibility factors frequently result in dysregulation of autophagic-lysosomal pathways. In particular, disrupted autophagy is implied in the accumulation of potentially neurotoxic products such as protein aggregates and their precursors and defective turnover of dysfunctional mitochondria. Here, we review the current state of iPSC-based assessment of autophagic dysfunction in the context of neurodegenerative disease modeling. The collected data show that iPSC technology is capable to reveal even subtle alterations in subcellular homeostatic processes, which form the molecular basis for disease manifestation.

Induced Pluripotent Stem Cells: Global Research Trends

The induced pluripotent stem cell (iPSC) was first described more than 10 years ago and is currently used in various basic science and clinical research fields. The aim of this report is to examine the trends in research using iPSCs over the last 10 years. The 2006-2016 PubMed database was searched using the MeSH term "induced pluripotent stem cells." Only original research articles were selected, with a total of 3323 articles. These were classified according to research theme into reprogramming, differentiation protocols for specific cells and/or tissues, pathophysiological research on diseases, and discovery of new drugs, and then the trends over the years were analyzed. We also focused on 232 research publications on the pathophysiological causes of diseases and drug discovery with impact factor (IF; Thomson Reuters) of six or more. The IF of each article was summed up by year, by main target disease, and by country, and the total IF score was expressed as trends of research. The trends of research activities of reprogramming and differentiation on specific cells and/or tissues reached maxima in 2013/2014. On the other hand, research on pathophysiology and drug discovery increased continuously. The 232 articles with IF ≥6 dealt with neurological, immunological/hematological, cardiovascular, and digestive tract diseases, in that order. The majority of articles were published from the United States, followed by Japan, Germany, and United Kingdom. In conclusion, iPSCs have become a general tool for pathophysiological research on disease and drug discovery.

Paving the Way Toward Complex Blood-Brain Barrier Models Using Pluripotent Stem Cells

A tissue with great need to be modeled in vitro is the blood-brain barrier (BBB). The BBB is a tight barrier that covers all blood vessels in the brain and separates the brain microenvironment from the blood system. It consists of three cell types [neurovascular unit (NVU)] that contribute to the unique tightness and selective permeability of the BBB and has been shown to be disrupted in many diseases and brain disorders, such as vascular dementia, stroke, multiple sclerosis, and Alzheimer's disease. Given the progress that pluripotent stem cells (PSCs) have made in the past two decades, it is now possible to produce many cell types from the BBB and even partially recapitulate this complex tissue in vitro. In this review, we summarize the most recent developments in PSC differentiation and modeling of the BBB. We also suggest how patient-specific human-induced PSCs could be used to model BBB dysfunction in the future. Lastly, we provide perspectives on how to improve production of the BBB in vitro, for example by improving pericyte differentiation protocols and by better modeling the NVU in the dish.

Cellular models as tools for the study of the role of alpha-synuclein in Parkinson's disease

Neurodegenerative diseases are highly debilitating conditions characterised primarily by progressive neuronal loss and impairment of the nervous system. Parkinson's disease (PD) is one of the most common of these disorders, affecting 1-2% of the population above the age of 65. Although the underlying mechanisms of PD have been extensively studied, we still lack a full understanding of the molecular underpinnings of the disease. Thus, the in vitro and in vivo models currently used are able to only partially recapitulate the typical phenotypes of the disease. Here, we review various cell culture models currently used to study the molecular basis of PD, with a focus on alpha-synuclein-associated molecular pathologies. We also discuss how different cell models may constitute powerful tools for high-throughput screening of molecules capable of modulating alpha-synuclein toxicity.

Driver or Passenger: Epigenomes in Alzheimer’s Disease

Alzheimer’s disease (AD) is a fatal neurodegenerative disease which is on the rise worldwide. Despite a wealth of information, genetic factors contributing to the emergence of AD still remain incompletely understood. Sporadic AD is polygenetic in nature and is associated with various environmental risks. Epigenetic mechanisms are well-recognized in the mediation of gene environment interactions, and analysis of epigenetic changes at the genome scale can offer new insights into the relationship between brain epigenomes and AD. In fact, recent epigenome-wide association studies (EWAS) indicate that changes in DNA methylation are an early event preceding clinical manifestation and are tightly associated with AD neuropathology. Further, candidate genes from EWAS interact with those from genome-wide association studies (GWAS) that can undergo epigenetic changes in their upstream gene regulatory elements. Functionally, AD-associated DNA methylation changes partially influence transcription of candidate genes involved in pathways relevant to AD. The timing of epigenomic changes in AD together with the genes affected indicate a critical role, however, further mechanistic insight is required to corroborate this hypothesis. In this respect, recent advances in neuronal reprogramming of patient-derived cells combined with new genome-editing techniques offer unprecedented opportunities to dissect the functional and mechanistic role of epigenomic changes in AD.

Recapitulating and Correcting Marfan Syndrome in a Cellular Model

Marfan syndrome (MFS) is a connective tissue disorder caused by mutations in FBN1 gene, which encodes a key extracellular matrix protein FIBRILLIN-1. The haplosufficiency of FBN1 has been implicated in pathogenesis of MFS with manifestations primarily in cardiovascular, muscular, and ocular tissues. Due to limitations in animal models to study the late-onset diseases, human pluripotent stem cells (PSCs) offer a homogeneic tool for dissection of cellular and molecular pathogenic mechanism for MFS in vitro. Here, we first derived induced PSCs (iPSCs) from a MFS patient with a FBN1 mutation and corrected the mutation, thereby generating an isogenic "gain-of-function" control cells for the parental MFS iPSCs. Reversely, we knocked out FBN1 in both alleles in a wild-type (WT) human embryonic stem cell (ESC) line, which served as a loss-of-function model for MFS with the WT cells as an isogenic control. Mesenchymal stem cells derived from both FBN1-mutant iPSCs and -ESCs demonstrated reduced osteogenic differentiation and microfibril formation. We further demonstrated that vascular smooth muscle cells derived from FBN1-mutant iPSCs showed less sensitivity to carbachol as demonstrated by contractility and Ca²⁺ influx assay, compared to the isogenic controls cells. These findings were further supported by transcriptomic anaylsis of the cells. Therefore, this study based on both gain- and loss-of-function approaches confirmed the pathogenetic role of FBN1 mutations in these MFS-related phenotypic changes.

Human stem cell modeling in neurofibromatosis type 1 (NF1)

The future of precision medicine is heavily reliant on the use of human tissues to identify the key determinants that account for differences between individuals with the same disorder. This need is exemplified by the neurofibromatosis type 1 (NF1) neurogenetic condition. As such, individuals with NF1 are born with a germline mutation in the NF1 gene, but may develop numerous distinct neurological problems, ranging from autism and attention deficit to brain and peripheral nerve sheath tumors. Coupled with accurate preclinical mouse models, the availability of NF1 patient-derived induced pluripotent stem cells (iPSCs) provides new opportunities to define the critical factors that underlie NF1-associated nervous system disease pathogenesis and progression. In this review, we discuss the generation and potential applications of iPSC technology to the study of NF1.

Induced pluripotent stem cells from patients with focal cortical dysplasia and refractory epilepsy

Focal cortical dysplasia (FCD) is caused by numerous alterations, which can be divided into abnormalities of the cortical architecture and cytological variations; however, the exact etiology of FCD remains unknown. The generation of induced pluripotent stem cells (iPSCs) from the cells of patients with neurological diseases, and their subsequent tissue‑specific differentiation, serves as an invaluable source for testing and studying the initial development and subsequent progression of diseases associated with the central nervous system. A total of 2 patients demonstrating seizures refractory to drug treatment, characterized as FCD Type IIb, were enrolled in the present study. Fibroblasts were isolated from residual skin fragments obtained from surgical treatment and from brain samples obtained during surgical resection. iPSCs were generated following exposure of fibroblasts to viral vectors containing POU class 5 homeobox 1 (OCT4), sex determining region Y‑box 2 (SOX2), Kruppel‑like factor 4 and c‑MYC genes, and were characterized by immunohistochemical staining for the pluripotent markers homeobox protein NANOG, SOX2, OCT4, TRA1‑60 and TRA1‑81. The brain samples were tested with antibodies against protein kinase B (AKT), phosphorylated‑AKT, mechanistic target of rapamycin (mTOR) and phosphorylated‑mTOR. Analysis of the AKT/mTOR pathway revealed a statistically significant difference between the cerebral tissues of the two patients, which were of different ages (45 and 12 years old). Clones with the morphological features of embryonic cells were detected on the 13th day and were characterized following three subcultures. The positive staining characteristics of the embryonic cells confirmed the successful generation of iPSCs derived from the patients' fibroblasts. Therefore, the present study presents a method to obtain a useful cellular source that may help to understand embryonic brain development associated with FCD.

LRRK2: An Emerging New Molecule in the Enteric Neuronal System That Quantitatively Regulates Neuronal Peptides and IgA in the Gut

BACKGROUND

Leucine-rich repeat kinase 2 (LRRK2) is a recently discovered molecule associated with familial and sporadic Parkinson's disease. It regulates many central neuronal functions such as cell proliferation, apoptosis, autophagy, and axonal extension. However, in contrast to the well-documented function of LRRK2 in central neurons, it is unclear whether LRRK2 is expressed in enteric neurons and affects the physiology of the gut.

AIMS

By examining LRRK2-KO mice, this study investigated whether enteric neurons express LRRK2 and whether intestinal neuronal peptides and IgA are quantitatively changed.

METHODS

Intestinal protein lysates and sections prepared from male C57BL/6 J mice were analyzed by Western blotting and immunostaining using anti-LRRK2 antibody, respectively. Intestinal neuronal peptide-mRNAs were quantified by real-time PCR in wild-type mice and LRRK2-KO mice. Intestinal IgA was quantified by ELISA. Lamina propria mononuclear cells (LPMCs) were analyzed by flow cytometry to evaluate the ratio of B1 to B2 B cells.

RESULTS

Western analysis and immunostaining revealed that LRRK2 is expressed in enteric neurons. The amounts of mRNA for vasoactive intestinal peptide, neuropeptide Y, and substance P were increased in LRRK2-KO mice accompanied by an increment of IgA. However, the intestinal B cell subpopulations were not altered in LRRK2-KO mice.

CONCLUSIONS

For the first time, we have revealed that LRRK2 is expressed in enteric neurons and related to quantitative alterations of neuronal peptide and IgA. Our study highlights the importance of LRRK2 in enteric neurons as well as central neurons.

Nurr1:RXRα heterodimer activation as monotherapy for Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by the loss of dopaminergic (DAergic) neurons in the substantia nigra and the gradual depletion of dopamine (DA). Current treatments replenish the DA deficit and improve symptoms but induce dyskinesias over time, and neuroprotective therapies are nonexistent. Here we report that Nuclear receptor-related 1 (Nurr1):Retinoid X receptor α (RXRα) activation has a double therapeutic potential for PD, offering both neuroprotective and symptomatic improvement. We designed BRF110, a unique in vivo active Nurr1:RXRα-selective lead molecule, which prevents DAergic neuron demise and striatal DAergic denervation in vivo against PD-causing toxins in a Nurr1-dependent manner. BRF110 also protects against PD-related genetic mutations in patient induced pluripotent stem cell (iPSC)-derived DAergic neurons and a genetic mouse PD model. Remarkably, besides neuroprotection, BRF110 up-regulates tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase (AADC), and GTP cyclohydrolase I (GCH1) transcription; increases striatal DA in vivo; and has symptomatic efficacy in two postneurodegeneration PD models, without inducing dyskinesias on chronic daily treatment. The combined neuroprotective and symptomatic effects of BRF110 identify Nurr1:RXRα activation as a potential monotherapeutic approach for PD.

Editing the genome of hiPSC with CRISPR/Cas9: disease models

The advent of human-induced pluripotent stem cell (hiPSC) technology has provided a unique opportunity to establish cellular models of disease from individual patients, and to study the effects of the underlying genetic aberrations upon multiple different cell types, many of which would not normally be accessible. Combining this with recent advances in genome editing techniques such as the clustered regularly interspaced short palindromic repeat (CRISPR) system has provided an ability to repair putative causative alleles in patient lines, or introduce disease alleles into a healthy “WT” cell line. This has enabled analysis of isogenic cell pairs that differ in a single genetic change, which allows a thorough assessment of the molecular and cellular phenotypes that result from this abnormality. Importantly, this establishes the true causative lesion, which is often impossible to ascertain from human genetic studies alone. These isogenic cell lines can be used not only to understand the cellular consequences of disease mutations, but also to perform high throughput genetic and pharmacological screens to both understand the underlying pathological mechanisms and to develop novel therapeutic agents to prevent or treat such diseases. In the future, optimising and developing such genetic manipulation technologies may facilitate the provision of cellular or molecular gene therapies, to intervene and ultimately cure many debilitating genetic disorders.

Drug discovery using induced pluripotent stem cell models of neurodegenerative and ocular diseases

The revolution of induced pluripotent stem cell (iPSC) technology provides a platform for development of cell therapy, disease modeling and drug discovery. Recent technological advances now allow us to reprogram a patient's somatic cells into induced pluripotent stem cells (iPSCs). Together with methods to differentiate these iPSCs into disease-relevant cell types, we are now able to model disease in vitro using iPSCs. Importantly, this represents a robust in vitro platform using patient-specific cells, providing opportunity for personalized precision medicine. Here we provide a review of advances using iPSC for drug development, and discuss the potential and limitations of iPSCs for drug discovery in neurodegenerative and ocular diseases. Emerging technologies that can facilitate the search for new drugs by assessment using in vitro disease models will also be discussed, including organoid differentiation, organ-on-chip, direct reprogramming and humanized animal models.

Studying human disease using human neurons

Utilizing patient derived cells has enormous promise for discovering new drugs for diseases of the nervous system, a goal that has been historically quite challenging. In this review, we will outline the potential of human stem cell derived neuron models for assessing therapeutics and high-throughput screening and compare to more traditional drug discovery strategies. We summarize recent successes of the approach and discuss special considerations for developing human stem cell based assays. New technologies, such as genome editing, offer improvements to help overcome the challenges that remain. Finally, human neurons derived from patient cells have advantages for translational research beyond drug screening as they can also be used to identify individual efficacy and safety prior to clinical testing and for dissecting disease mechanisms. This article is part of a Special Issue entitled SI: Exploiting human neurons.

Transcriptomic profiling of purified patient-derived dopamine neurons identifies convergent perturbations and therapeutics for Parkinson's disease

While induced pluripotent stem cell (iPSC) technologies enable the study of inaccessible patient cell types, cellular heterogeneity can confound the comparison of gene expression profiles between iPSC-derived cell lines. Here, we purified iPSC-derived human dopaminergic neurons (DaNs) using the intracellular marker, tyrosine hydroxylase. Once purified, the transcriptomic profiles of iPSC-derived DaNs appear remarkably similar to profiles obtained from mature post-mortem DaNs. Comparison of the profiles of purified iPSC-derived DaNs derived from Parkinson's disease (PD) patients carrying LRRK2 G2019S variants to controls identified significant functional convergence amongst differentially-expressed (DE) genes. The PD LRRK2-G2019S associated profile was positively matched with expression changes induced by the Parkinsonian neurotoxin rotenone and opposed by those induced by clioquinol, a compound with demonstrated therapeutic efficacy in multiple PD models. No functional convergence amongst DE genes was observed following a similar comparison using non-purified iPSC-derived DaN-containing populations, with cellular heterogeneity appearing a greater confound than genotypic background.

Induced pluripotent stem cell technology: a decade of progress

Since the advent of induced pluripotent stem cell (iPSC) technology a decade ago, enormous progress has been made in stem cell biology and regenerative medicine. Human iPSCs have been widely used for disease modelling, drug discovery and cell therapy development. Novel pathological mechanisms have been elucidated, new drugs originating from iPSC screens are in the pipeline and the first clinical trial using human iPSC-derived products has been initiated. In particular, the combination of human iPSC technology with recent developments in gene editing and 3D organoids makes iPSC-based platforms even more powerful in each area of their application, including precision medicine. In this Review, we discuss the progress in applications of iPSC technology that are particularly relevant to drug discovery and regenerative medicine, and consider the remaining challenges and the emerging opportunities in the field.

Genome-Scale Networks Link Neurodegenerative Disease Genes to α-Synuclein through Specific Molecular Pathways

Numerous genes and molecular pathways are implicated in neurodegenerative proteinopathies, but their inter-relationships are poorly understood. We systematically mapped molecular pathways underlying the toxicity of alpha-synuclein (α-syn), a protein central to Parkinson's disease. Genome-wide screens in yeast identified 332 genes that impact α-syn toxicity. To "humanize" this molecular network, we developed a computational method, TransposeNet. This integrates a Steiner prize-collecting approach with homology assignment through sequence, structure, and interaction topology. TransposeNet linked α-syn to multiple parkinsonism genes and druggable targets through perturbed protein trafficking and ER quality control as well as mRNA metabolism and translation. A calcium signaling hub linked these processes to perturbed mitochondrial quality control and function, metal ion transport, transcriptional regulation, and signal transduction. Parkinsonism gene interaction profiles spatially opposed in the network (ATP13A2/PARK9 and VPS35/PARK17) were highly distinct, and network relationships for specific genes (LRRK2/PARK8, ATXN2, and EIF4G1/PARK18) were confirmed in patient induced pluripotent stem cell (iPSC)-derived neurons. This cross-species platform connected diverse neurodegenerative genes to proteinopathy through specific mechanisms and may facilitate patient stratification for targeted therapy.

PINK1, Parkin, and Mitochondrial Quality Control: What can we Learn about Parkinson's Disease Pathobiology?

The first clinical description of Parkinson's disease (PD) will embrace its two century anniversary in 2017. For the past 30 years, mitochondrial dysfunction has been hypothesized to play a central role in the pathobiology of this devastating neurodegenerative disease. The identifications of mutations in genes encoding PINK1 (PTEN-induced kinase 1) and Parkin (E3 ubiquitin ligase) in familial PD and their functional association with mitochondrial quality control provided further support to this hypothesis. Recent research focused mainly on their key involvement in the clearance of damaged mitochondria, a process known as mitophagy. It has become evident that there are many other aspects of this complex regulated, multifaceted pathway that provides neuroprotection. As such, numerous additional factors that impact PINK1/Parkin have already been identified including genes involved in other forms of PD. A great pathogenic overlap amongst different forms of familial, environmental and even sporadic disease is emerging that potentially converges at the level of mitochondrial quality control. Tremendous efforts now seek to further detail the roles and exploit PINK1 and Parkin, their upstream regulators and downstream signaling pathways for future translation. This review summarizes the latest findings on PINK1/Parkin-directed mitochondrial quality control, its integration and cross-talk with other disease factors and pathways as well as the implications for idiopathic PD. In addition, we highlight novel avenues for the development of biomarkers and disease-modifying therapies that are based on a detailed understanding of the PINK1/Parkin pathway.

Integrating Gene Correction in the Reprogramming and Transdifferentiation Processes: A One-Step Strategy to Overcome Stem Cell-Based Gene Therapy Limitations

The recent advent of induced pluripotent stem cells (iPSCs) and gene therapy tools has raised the possibility of autologous cell therapy for rare genetic diseases. However, cellular reprogramming is inefficient in certain diseases such as ataxia telangiectasia, Fanconi anemia, LIG4 syndrome, and fibrodysplasia ossificans progressiva syndrome, owing to interference of the disease-related genes. To overcome these therapeutic limitations, it is necessary to fundamentally correct the abnormal gene during or prior to the reprogramming process. In addition, as genetic etiology of Parkinson's disease, it has been well known that induced neural stem cells (iNSCs) were progressively depleted by LRRK2 gene mutation, LRRK2 (G2019S). Thus, to maintain the induced NSCs directly derived from PD patient cells harboring LRRK2 (G2019S), it would be ideal to simultaneously treat the LRRK2 (G2019S) fibroblast during the process of TD. Therefore, simultaneous reprogramming (or TD) and gene therapy would provide the solution for therapeutic limitation caused by vulnerability of reprogramming or TD, in addition to being suitable for general application to the generation of autologous cell-therapy products for patients with genetic defects, thereby obviating the need for the arduous processes currently required.

LRRK2 Expression Is Deregulated in Fibroblasts and Neurons from Parkinson Patients with Mutations in PINK1

Mutations in PINK1 (PARK6), a serine/threonine kinase involved in mitochondrial homeostasis, are associated with early onset Parkinson’s disease. Fibroblasts from Parkinson’s disease patients with compound heterozygous mutations in exon 7 (c.1488 + 1G > A; c.1252_1488del) showed no apparent signs of mitochondrial impairment. To elucidate changes primarily caused by lack of functional PINK1, we over-expressed wild-type PINK1, which induced a significant downregulation of LRRK2 (PARK8). Indeed, we found that LRRK2 protein basal levels were significantly higher in the mutant PINK1 fibroblasts. To examine the interaction between the two PARK genes in a disease-relevant cell context, we generated induced pluripotent stem cell (iPSC) lines from mutant, carrier and control fibroblasts by lentiviral-mediated re-programming. Efficiency of neural induction and dopamine differentiation using a floor-plate induction protocol was similar in all genotypes. As observed in fibroblasts, PINK1 mutant neurons showed increased LRRK2 expression both at the RNA and protein level and transient over-expression of wild-type PINK1 efficiently downregulated LRRK2 levels. Additionally, we confirmed a dysregulation of LRRK2 expression in fibroblasts from patients with a different homozygous mutation in PINK1 exon 4, c.926G > A (G309D). Thus, our results identify a novel role of PINK1 modulating the levels of LRRK2 in Parkinson’s disease fibroblasts and neurons, suggest a convergent pathway for these PARK genes, and broaden the role of LRRK2 in the pathogenesis of Parkinson’s disease.

LRRK2 at the interface of autophagosomes, endosomes and lysosomes

Over the past 20 years, substantial progress has been made in identifying the underlying genetics of Parkinson's disease (PD). Of the known genes, LRRK2 is a major genetic contributor to PD. However, the exact function of LRRK2 remains to be elucidated. In this review, we discuss how familial forms of PD have led us to hypothesize that alterations in endomembrane trafficking play a role in the pathobiology of PD. We will discuss the major observations that have been made to elucidate the role of LRRK2 in particular, including LRRK2 animal models and high-throughput proteomics approaches. Taken together, these studies strongly support a role of LRRK2 in vesicular dynamics. We also propose that targeting these pathways may not only be beneficial for developing therapeutics for LRRK2-driven PD, but also for other familial and sporadic cases.